Image intensifier devices employ a photocathode for conversion of photons to electrons, a microchannel plate for electron multiplication, and a phosphor-coated anode to convert electrons back to photons. The microchannel plate image intensifier is currently manufactured in two types that are commonly referred to as generation II and generation III type image tubes. The primary difference between these two types of image intensifiers lies in the type of photocathode employed. Generation II image intensifier tubes have a multi-alkali photocathode with a spectral sensitivity in the range of 400-900 nannometers. This spectral range can be extended to the blue or red by modification of the multi-alkali composition and/or thickness. Generation III image intensifier tubes have a p-doped gallium arsenide (GaAs) photocathode that has been activated to negative electron affinity (NEA) by the adsorption of cesium and oxygen on the surface. This material has approximately twice the quantum efficiency of the generation II photocathode. An extension of the spectral response to the near infrared can be accomplished by alloying indium with gallium arsenide.
Existing photocathodes have several disadvantages. Generation III photocathodes are generally made using expensive processes such as metal/organic/chemical/vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Besides being expensive, processes such as the MOCVD process use toxic chemicals which must be carefully controlled to avoid harming the people manufacturing the photocathodes.
Generation III photocathodes are normally heat cleaned to remove surface oxides and contaminants just prior to activation and seal in an evacuated image-intensifier tube. Small leaks in such tubes will sometimes prevent a vacuum from forming and the tube will be unusable. If a proper vacuum seal is not formed or if a leak develops, one can normally not attempt to seal a generation III photocathode in a different image intensifier tube. Gallium arsenide photocathodes often suffer lattice damage when heated a second time to remove surface contaminants rendering the cathode unusable.
Existing generation III photocathodes are also sensitive to lasers. Direct contact of a laser beam on a generation III photocathode ordinarily destroys the photocathode. Sensitivity to laser energy is a drawback when night vision equipment is being used for military operations as many modern weapon systems use lasers.